For many years, oral inhalation delivery of drug-laden aerosols to the lungs has been an effective means of drug delivery. One type of device that is effective for delivering particulate drugs is a metered dose inhaler, which includes a pressurized canister with a metering valve that contains a drug formulation. In “press and breathe” versions of metered dose inhalers, the canister is placed within an actuator comprised of a housing that covers a lower portion of the canister, leaving the top portion exposed. The metering valve seats into a sump/orifice assembly inside the base of the housing. The orifice is positioned at an acute angle to the valve stem and directs discharge of the particulate drug formulation approximately through a conduit attached to the housing at a 90 degree angle and terminating in a mouthpiece. To administer the drug, the user seals his/her lips around the mouthpiece of the device and simultaneously inhales (an inspiratory breath) while depressing the exposed portion of the canister into the housing. The canister translates downward in a manner which actuates the metering valve and thus causes release of the drug as an aerosol plume which is then drawn into the respiratory tract as the user inhales. It can be difficult for some users to coordinate the release of the aerosol plume with their inspiratory breath.
In order to address problems with “press and breathe” actuated inhalers, improved inhaler devices have been developed that release the aerosol plume of drug automatically when the user takes in an inspiratory breath. These are termed “breath-actuated pressurized meter dose inhalers” (“BApMDI”s). In exemplary BApMDI devices, actuation can be carried out using a spring that is compressed by opening a cover. This spring energy is stored until the BApMDI is triggered by the user's breath, at which time the spring force is applied to depress the pressurized canister and cause the release of a plume of aerosolized medication into the users' breath. Such triggering mechanisms depend upon an eductor element that includes a venturi having a flow path that narrows in a constriction zone and serves to increase the local velocity of the flow of inspiratory breath to create a siphon suitable to actuate the device.
Although BApMDI devices represent an improvement in the art, it has been found that the energy (pressure drop) required to trigger the spring energy (and thus actuate the device) may exceed that which can be applied by a normal human breath. This can particularly present a problem where the user has compromised lung function, or where the user is an adolescent without the lung capacity of an adult. There accordingly remains a need in the art to provide improved BApMDI devices with a triggering mechanism that enables actuation with a normal human breath.